A helicopter can stall if its airspeed is too high. This is referred to as a retreating blade stall. In forward flight, a helicopter's retreating blade (the blade moving in the opposite direction as the aircraft) has a lower airspeed than its advancing blade. A retreating blade stall occurs when the retreating blade exceeds its critical angle of attack. As the helicopter's airspeed increases, the retreating blade's airspeed decreases and angle of attack increases. A stall occurs when the helicopter's airspeed exceeds an upper limit corresponding to the retreating blade's critical angle of attack.
The upper limit of helicopter airspeed is influenced by a number of factors. Some of these factors are outside of a pilot's control. For example, a change in ambient temperature near the aircraft can change the airspeed upper limit. In some cases, a pilot may maintain the same flight path and speed, but a stall may occur because of such external factors. Such a stall can be disorienting for a pilot, not just because of the loss of control of the helicopter, but also because the stall occurs without a warning to the pilot.
Stall warnings are available for fixed-wing aircraft, but not for helicopters. Fixed wing pilots and helicopter pilots can calculate a stall speed, but fixed wing pilots have an advantage because their calculations (and memory of their calculations) are supplemented by active warning systems in the aircraft. This may explain why helicopters' safety record is inferior to fixed wing aircraft's record.